Different climatic conditions are produced by the geography and uneven heating of the Earth. Plant and animal forms that are characteristic of a particular geographic area with a common climate constitute biomes. Each biome is characterized by specific climax communities. All the biomes together form the biosphere.

The various biotic and abiotic factors at play on Earth result in six major terrestrial biomes. Terrestrial biomes are categorized according to the types of plants they support. The fundamental characteristics of each type are described in the list below.

Rain forests have the highest rainfall of all biomes (100–180 inches per year), which results in the greatest animal and plant diversity. Trees form canopies that block sunlight from reaching the ground. Most animal species live in the canopy, while the forest floor is inhabited predominantly by insects and saprophytes and consists of soil low in nutrients. Decomposed products on the forest floor are washed away or quickly reabsorbed by plants. Tropical rain forests can be found in Central America, the Amazon basin in South America, Central Africa, and Southeast Asia.

This biome is characterized by grassland with sparse trees, with extended dry periods or droughts. Tropical savannas generally border rain forests and receive a yearly total of 40 to 60 inches of rainfall. They support large herbivores, such as antelope, zebra, elephants, and giraffe. Most tropical savannas exist in Africa. Temperate savannas, such as the Pampas in Argentina and the prairies east of the Rocky Mountains in the United States, receive only about 10 to 30 inches of rain a year. Grasses and shrubs dominate the landscape and support insects, birds, smaller burrowing animals, and larger, hoofed animals such as bison.

Deserts are the driest biome, receiving less than 10 inches of rain per year. They exhibit radical temperature changes between day and night. Animals of the desert such as lizards, snakes, birds, and insects are typically small and have adapted to the dry, hot climate by being nocturnally active. Plants, such as cactus, have evolved waxy cuticles, fewer stomata, spiky leaves, and seeds capable of remaining dormant until sufficient resources are available. Deserts exist in Asia, Africa, and North America.

Rainfall in temperate deciduous forests is evenly distributed throughout the year. The biome has distinct summer and winter seasons. It has long growing seasons during the summer. In winter, the deciduous plants drop their leaves and enter a period of dormancy. Beech and maple dominate in colder variations of this biome, while oak and hickory are more prevalent where temperatures are warmer. Animals in deciduous forests are both herbivorous and carnivorous, such as deer, fox, owl, and squirrel. The forest floor is fertile and contains fungi and worms. Temperate deciduous forests exist mainly on the east coast of North America and in central Europe.

The taiga is a forest biome but is colder and receives less rainfall than deciduous forests. Coniferous (cone-bearing) trees, especially spruce, dominate the taiga. The trees also have needle-shaped leaves that help conserve water. Taiga forests sustain birds, small mammals such as squirrels, large herbivorous mammals such as moose and elk, and large carnivorous mammals such as wolves and grizzly bears. Taiga exist mainly in Russian and northern Canada.

This biome is located in the far north and is covered by ice sheets for the majority of the year. The soil, down to a few feet, remains permanently frozen, though in the summer, the topsoil can melt and support a short growing season. Very few plants grow in the northernmost parts of the tundra, but lichens, mosses, and grasses occupy some more southern areas. Animals must be well suited for extreme cold or must migrate. The tundra supports large herbivores such as reindeer and caribou, large predators such as bear, and some birds.

Aquatic biomes account for 70 percent of the Earth’s surface and contain the majority of plant and animal life. Aquatic biomes also account for a vast portion of the photosynthesis, and therefore oxygen production, that occurs on Earth. There are two types of aquatic biomes, based on the type of water found in each: marine and freshwater.

Marine biomes refer to the oceans that all connect to form a single, great body of water. Since water has an immense capacity to absorb heat with little temperature increase, conditions remain uniform over these large aquatic bodies. Marine biomes are divided into three zones: intertidal/littoral, neritic, and pelagic.

The intertidal zone, also called the littoral zone, is the region where land and water meet. It experiences periodic dryness with changing tides and is inhabited by algae, sponges, various mollusks, starfish, and crabs.

The neritic zone extends to 600 feet beneath the water’s surface and sits on the continental shelf, hundreds of miles from shores. Algae, crustaceans, and numerous fish inhabit this region.

The pelagic zone consists of a photic zone (reaching 600 feet below sea level) and below that an aphotic zone. Light penetrates the photic zone, which is why it contains photosynthetic plankton. The photic zone also is home to heterotrophs such as bony fish, sharks, and whales that prey on these producers as well as on each other. No light penetrates the aphotic zone, which is a kind of watery circus of the bizarre, where extreme cold water, darkness, and high pressure have spurred strange evolutionary paths. The region is home to some chemosynthetic autotrophs. Other denizens of the deep are scavengers that feed on dead organic matter falling from the higher realms and predators who feed on each other.

Freshwater biomes include rivers, lakes, and marshes. Life here is affected by temperature, salt concentration, light penetration, depth, and availability of dissolved CO₂ and O₂. Freshwater biomes are much smaller than marine biomes, so conditions are less stable. Organisms that live in these regions must be able to handle the greater extremes. The very nature of freshwater also demands special characteristics of the organisms that live within it. In freshwater environments, the salt concentration within the cell of an organism is higher than the salt concentration in the water. A concentration exists between the interior of cells and the exterior environment: water from the environment is constantly diffusing into the organism. Organisms in freshwater need homeostatic systems to maintain proper water balance.